Abstract

A simple and versatile method for the synthesis of 1,5-benzodiazepines is via condensation of o-phenylenediamines (OPDA) and ketones in the presence of catalytic amount of H-MCM-22 using acetonitrile as solvent at room temperature. In all the cases, the reactions are highly selective and are completed within 1–3 h. The method is applicable to both cyclic and acyclic ketones without significant differences. The reaction proceeds efficiently under ambient conditions with good-to-excellent yields.

1. Introduction

Benzodiazepines constitute an important class of biologically active compounds and their synthesis has been receiving much attention in the field of medicinal and pharmaceutical chemistry owing to their application as anticonvulsant, anti-inflammatory, analgesic, hypnotic, and sedative agents and to their hypnotic activity [1–6]. The derivatives of 1,5-benzodiazepines are also used as dyes for acrylic fibers in photography [7]. In addition, benzodiazepines are the useful precursors for the synthesis of other fused ring compounds such as triazolo-, oxadiazolo-, oxazino-, or furano-benzodiazepines [8–11]. Benzodiazepines are generally synthesized by the condensation of o-phenylenediamine (OPDA) with a,b-unsaturated carbonyl compounds, b-haloketones, or with ketones [12] using acidic catalysts which are critical to enhance the condensation process. Different reagents such as BF3-etherate, polyphosphoric acid, NaBH4, MgO/POCl3, Yb(OTf)3, Ga(OTf)3, lead nitrate, L-proline, acetic acid under microwave conditions, molecular iodine, and ionic liquids have also been used for the synthesis of benzodiazepines [13–23]. Recently the synthesis of benzodiazepines has also been reported using different solid acid catalysts such as sulfated zirconia, Al2O3/P2O5, Ag3PW12O40, PVP-FeCl3, and zeolite catalysts [24–28]. Unfortunately, many of these catalysts suffer from one or more limitations, such as long reaction times, occurrence of several side reactions, drastic reaction conditions, low yields, and tedious workup procedure. In addition, the solid oxide catalyst used previously had poor textural parameters such as low surface area and pore volume which do not support a better performance in the synthesis of benzodiazepines. These factors stimulate the search for a better catalyst, which should offer a high activity for the synthesis of 1,5-benzodiazepines under mild reaction conditions. In recent years, mesoporous materials with different structure and tunable pore diameters have been receiving much attention.

Here we demonstrate, the synthesis of 1,5-benzodiazepine using H-MCM-22 as catalyst through a condensation reaction between OPDA and ketones in acetonitrile condition. The effect of catalyst and the catalyst concentration on the above process has been investigated in detail.

2. Experimental Section

2.1. Synthesis of Catalysts Synthesis of (H-MCM-22)

The hydrothermal synthesis of layered aluminosilicate MCM-22 was carried out by using hexamethyleneimine as an organic template at dynamic conditions. A total of 3.110 g of anhydrous sodium aluminate (50–56% Al2O3, maximum 0.05% Fe2O3, 40–45% Na2O, purchased from Riedel-deHaën) and 1.930 g of sodium hydroxide (97%, from Merck) was dissolved in 415 g of deionized water. The solution was thoroughly stirred for 15 minutes, and then 25.4 g of hexamethyleneimine (HMI) (99%, from Aldrich) was added dropwise with vigorous stirring. To this, 30.7 g of silica (Aerosil 200, Degussa) was added in small portions and the resulting slurry continued to be stirred vigorously. After 30 minutes of stirring, at room temperature, a gel was formed; this was then placed in an autoclave and heated at 150°C for 10 days. After immersing the autoclaves in cold water, the resulting material was washed and centrifuged until pH ≤ 9 and subsequently dried at 70°C. A 1-gram sample of MCM-22 was calcined in the absence of air at 650°C for 7 hours, generating Na-MCM-22 zeolite. The process took place in an oven in order to remove water and organic precursor. The oven was programmed at a heating rate of 5°C/min until 650°C for 7 h. The Na-MCM-22 zeolite is converted into the H-form by the following procedure: 9.000 g of Na-MCM-22, 7.230 g of NH4Cl, and 13.80 mL of deionized water were mixed with 0.1 M hydrochloric acid solution, in order to reach pH 4.0. The mixture was stirred at 60°C for 30 min. Then the material was filtered under suction and washed with deionized water. After the removal of chlorides, the resulting material, NH4-MCM-22 zeolite, was placed in an oven at 60°C for 24 h. The ammonium form of MCM-22 was converted into H-form by calcination over 60 min at 200°C.

2.2. Characterization of the Catalysts and the Products

X-Ray Diffraction (XRD): the samples were sieved in an ABNT n° 200 (0.074 mm) sieve and then placed in an aluminum sample holder for X-ray diffraction assays, using a Shimadzu XRD 6000 equipment. The operational details of the technique were set as follows: copper Kα radiation at 40 KV/30 mA, with a goniometer speed of 2°/min and a step of 0.02° in the 2θ range scanning from 2° to 50°. The only d-spacings of interest in the X-ray patterns were the basal spacings along the 𝑐 axis.

Fourier-transform infrared (FT-IR) spectroscopy: the MCM-22 sample was submitted to physical treatment in accordance with the KBr method, which consists of mixing 0.007 g of the sample and 0.1 g of KBr, grinding and pressing the solid mixture to 5 ton for 30 s in order to form a pastille that allows the passage of light. Characterization was performed using an infrared spectrophotometer SHIMADZU FT-IR in the wavelength ranges from 2000 to 400 cm−1, with increments of 650 cm−1 and a resolution of 4 cm−1.

Characterization of the product was performed on SHIMADZU FT-IR spectrometer, and the sample was prepared with KBr and pressed into wafer/pellet. Spectra were collected in the range of 400–4000 cm−1. NMR of the products was performed on BRUKER ADVANCE II 300 NMR spectrometer.

Gas chromatography for benzodiazepine was performed using Younglin gas chromatograph (YL6100 GC). A narrow-bore capillary column (25 m × 0.22 with 0.25 μm thickness) was used and an oven temperature of 250°C was employed with injector and detector temperature of 2750C. Yield was calculated from the peak areaArea=Heightofpeak×Widthathalf-height,(1)
and % yield was calculated as%yield=AreaofcomponentTotalarea×100.(2)

2.3. Synthesis of 1,5-Benzodiazepine

A mixture of OPDA (1 mmol, 108.1 mg), ketone (2.5 mmol, 145.2 mg), and H-MCM-22 (100 mg) was stirred in acetonitrile (4 mL) at room temperature until thin layer chromatography indicated the reaction was completed (as shown in Scheme 1). Ethyl acetate (10%) in hexane was used as the mobile phase and both the reactant and the final product were spotted on the TLC plate. The product retention factor (Rf) was observed at 0.4. The disappearance of the reactant spot on the TLC place indicates the completion of the reaction. After completion of the reaction, ethyl acetate (20 mL) was added to the reaction mixture and the catalyst was recovered by filtration. The organic layer was concentrated and the crude product was purified by silica gel column chromatography using ethyl acetate: n-hexane (1 : 9) as eluent to afford the desired product.

3. Result and Discussion

The XRD patterns of samples of MCM-22 zeolite are in agreement with those already reported [30–32]. All acquired X-ray patterns identified the products as highly crystalline materials.

The FTIR spectrum of MCM-22 shows the presence of bands related to the symmetrical and asymmetrical stretching of O-H band. The band at 786 cm−1 refers to external symmetrical stretching, whilst the band at 1237 cm−1 refers to external asymmetric stretching. The bands at 1095 and 1070 cm−1 correspond to internal asymmetric stretching and the bands at 594 and 550 cm−1 are attributed to the presence of double rings in MCM-22 structure. At 1626 cm−1 the band is ascribed to the angular deformation of the N–H bond and water. The band at 449 cm−1 is related to the internal deformation of O–H bands [33]. The band centered at 1384 cm−1 indicates the presence of amine functional group (HMI).

Initially the focus was on the synthesis of 1,5-benzodiazepine. The reaction was carried out using H-MCM-22 through a simple condensation reaction between OPDA and acetone using acetonitrile as solvent at room temperature and the results are given in Table 2. The catalyst was found to be highly active, affording 87% isolated yield of 1,5-benzodiazepine in 60 min. It should also be noted that when the reaction was conducted without any catalyst, the reaction does not occur. These results indicate that the acidity of the catalyst plays a critical role in this transformation and dictates the activity of the catalyst. The role of Brönsted acid site of H-MCM-22 catalyst on the formation of 1,5-benzodiazepine and the reaction mechanism are clearly depicted in Scheme 2. As H-MCM-22 showed a much higher activity than other catalysts used in this study under the optimized reaction conditions, we have used H-MCM-22 for the remaining catalytic studies.

Scheme 2: Reaction mechanism of the synthesis of 1,5-benzodiazepines using H-MCM-22 catalyst at room temperature [29].

The synthesis of 1,5-benzodiazepines was also carried out over different amounts of H-MCM-22 at room temperature for 60 min and the results are given in Table 1. The weight of catalyst was increased between 50 and 200 mg. It was found that the yield increases from 30% to 87% with increase in the weight of the catalyst from 50 to 150 mg, respectively. This could be mainly due to the availability of huge number of acidic sites on the porous surface of H-MCM-22. It should be noted that the yield of the product remains constant with the further increase in the weight of the catalyst from 150 to 200 mg. Hence, we used the weight of the catalyst as 150 mg for rest of the studies.

Table 1: Effect of weight of H-MCM-22 on the synthesis of 1,5-benzodiazepine.

Table 2: Synthesis of 1,5-benzodiazepines and its derivatives using H-MCM-22 through a condensation reaction between a series of OPDA and various ketones.

The excellent catalytic performance of H-MCM-22 in the synthesis of 1,5-benzodiazepine stimulated us to extend this process for the synthesis of various derivatives of benzodiazepines using various substituted OPDAs and a series of symmetrical and unsymmetrical ketones and the results are shown in Table 2. In all the cases, the reactions are highly selective and are completed within 1–3 h. The catalyst showed excellent activity in all the cases, showing 65–87% isolated yield of the corresponding derivatives of 1,5-benzodiazepine. It was found that the catalyst showed superior performance with high yields in a relatively shorter reaction time than Ersorb-4 (E4), a clinoptylolite-type zeolite catalyst reported earlier [28]. It was interesting to see that E4 needed a high temperature and a longer reaction time conditions to achieve high isolated yield of the final product whereas H-MCM-22 was active even at room temperature. These findings reveal the superior nature of the studied catalyst in this transformation.

Chlorosubstituted OPDA and substituted ketones were also used with similar success to provide the corresponding benzodiazepines in high yields, which are of much interest with regard to biological activity. Chlorosubstituted benzodiazepines in good yields in the presence of this catalyst were achieved easily.

It is also important to note that the workup of the reaction mixture is very simple. The catalyst can be filtered out easily and the solvent was evaporated. Recycling experiments were conducted to find out the stability of the catalyst after the reaction. The catalyst was easily separated by centrifuge and reused after activation at 500°C for 3-4 h. The efficiency of the recovered catalyst was verified with the reaction of OPDA and Ketone (Entry 1). Using the fresh catalyst, the yield of product (2a) was 87%, while the recovered catalyst in the three subsequent recyclization gave the yields of 85%, 81%, and 75%, respectively. The small reduction in the catalytic activity after three reaction cycles may be mainly due to the loss of catalyst or catalyst structure during the recovery process. These results reveal that the catalyst can be recycled several times without much loss in its activity.

4. Conclusion

The work reported has used H-MCM-22 catalyst for the synthesis of various benzodiazepines by using substituted OPDA and a series of symmetrical and unsymmetrical ketones at room temperature. This method is quite simple and selective. The catalyst gave high isolated yield of the derivatives of 1,5-benzodiazepine in a shorter reaction time at room temperature and can be recycled several times. We strongly hope that the highly stable H-MCM-22 catalyst could pave the way for the production of 1,5-benzodiazepine and its derivatives and create the platform for the commercialization of the process by replacing the existing homogenous catalysts which suffered from various drawbacks such as corrosion, toxicity, waste production, and high cost.

Acknowledgments

Authors are highly thankful to DRDE Gwalior for financial assistance and SAIF Chandigarh for the characterization of the synthesized materials.